Integrated programmable gain amplifier circuit and system including the circuit

ABSTRACT

An integrated programmable gain amplifier circuit that receives at an input an analog signal, circuit including an operational amplifier and a gain setup network comprising resistive elements and selection elements, which may be controlled in order to setup the gain of the amplifier circuit. The gain setup network further includes capacitive elements, for defining, together with the resistive elements and the operational amplifier, an anti-aliasing filter of the active RC type.

BACKGROUND

1. Technical Field

The present disclosure refers to an integrated programmable gainamplifier circuit and to a system including such an integrated circuit.

2. Description of the Related Art

With reference to the block diagram shown in FIG. 1, transmission andrecording systems 1 are known in the field of mobile telephonyvoice/audio for receiving at input a microphone analog signal in orderto convert it into a digital signal. The system includes an integratedcircuit in which a programmable gain amplifier 2 (which in thisconfiguration is commonly called pre-amplifier), an anti-aliasing filter3, and an analog-digital converter 4 are provided.

In such transmission or recording systems 1, the pre-amplifier 2 is ofthe programmable gain type, in order to adapt the dynamics of themicrophone signal, depending on the particular type of microphone used,to the input dynamic of the analog-digital converter 4. The gain of thepre-amplifier 2 is set during system setup in order to provide the bestperforming interfacing of the analog-digital converter 4 to theparticular microphone model to be used in the system.

The anti-aliasing filter 3 has the task of eliminating the spuriouscomponents of the input microphone signal that surround the samplingfrequency of the analog-digital converter in order to avoid thesecomponents from being carried in the band used by the samplingoperation. Such a filter 3 is therefore a low pass filter.

The analog-digital converter 4 is provided for converting thepreamplified and filtered analog microphone signal into a digitalsignal, for example in order to store or transmit this digital signal.

In designing next-generation mobile communications devices, such ascellular telephone devices, for which audio performances will berequired, which may be similar to those of consumer hi-fi equipments,the need is felt for reducing the noise introduced in the digitalsamples by the path through the various blocks 2, 3, and 4 representedin the diagram of FIG. 1.

It has been observed that in order to achieve this goal, it is necessaryto develop integrated circuits with an increase of consumption andsurface area of the three blocks 2, 3 and 4 of FIG. 1, since the noisedecreases with the square root of the these two parameters. However,this solution is not practical since, in order to achieve the requiredperformances in terms of noise reduction, it would be necessary todesign mobile communications devices with an unacceptable level ofconsumption, although it is known that the battery consumption is a veryimportant performance factor, and it would be necessary to developintegrated circuits that are not competitive in terms of silicon arearequirements.

BRIEF SUMMARY

The present disclosure provides a circuit that provides a sufficientnoise reduction while at the same time not requiring an increase inconsumption and area or requiring only a relatively limited consumptionand area increase.

In accordance with one embodiment, an integrated programmable gainamplifier integrate circuit as defined in the claims is provided.

In accordance with one embodiment, an integrated programmable gainamplifier circuit for receiving at an input an analog signal isprovided. The circuit includes an input for receiving an analog signal,the circuit comprising an operational amplifier and a gain setup networkcomprising resistive elements and selection elements, that arecontrolled to setup gain of the amplifier circuit, the gain setupnetwork including capacitive elements, for defining, together with theresistive elements and the operational amplifier, an anti-aliasingfilter of the active RC type.

In accordance with another embodiment of the present disclosure, acircuit is provided that includes an anti-aliasing filter of the activeRC type that includes an amplifier; and a gain network that includes aninput network coupled to a first input of the amplifier, and a feedbacknetwork coupled between an output of the amplifier and a second input ofthe amplifier, the feedback network comprising a plurality of RC cellscoupled in cascode, each RC cell associated with a respective selectionswitch to selectively couple each of the plurality of RC cells in thecascode connection.

In accordance with yet a further embodiment of the present disclosure, amobile communications device is provided that includes an anti-aliasingfilter of the active RC type that includes an amplifier; and a gainnetwork that includes an input network coupled to a first input of theamplifier, and a feedback network coupled between an output of theamplifier and a second input of the amplifier, the feedback networkcomprising a plurality of RC cells coupled in cascode, each RC cellassociated with a respective selection switch to selectively couple eachof the plurality of RC cells in the cascode connection.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The disclosure will be better understood from the following detaileddescription of one preferred embodiment, which is illustrative andtherefore in no way limiting with respect to the appended drawings,wherein:

FIG. 1 shows a block diagram of a known system for processing amicrophone signal;

FIG. 2 shows a block diagram of a system for processing a microphonesignal that includes an integrated amplifier and filter circuit and ananalog-digital converter in accordance with the present disclosure;

FIG. 3 shows the circuit diagram of an integrated programmable gainamplifier and filter circuit to be used in the system of FIG. 2;

FIG. 4 shows a circuit diagram of an integrated programmable gainamplifier and filter circuit to be used in the system of FIG. 2;

FIG. 5 shows the circuit diagram of an integrated programmable gainamplifier and filter circuit to be used in the system of FIG. 2; and

FIG. 6 shows the circuit diagram of an integrated programmable gainamplifier and filter circuit to be used in the system of FIG. 2.

In the various figures, same or like elements are indicated by the samereference numerals.

DETAILED DESCRIPTION

With reference to FIG. 2, a system 20 for processing an analog signalS_(MC) for amplifying, filtering and converting the signal S_(MC) fromanalog to digital is illustrated in schematic form.

According to a preferred non limiting embodiment, the processing system20 is a system for transmission or recording or both transmission andrecording or receiving an analog voice/audio signal S_(MC). In thiscase, the analog voice/audio signal S_(MC) is for example an analogsignal provided as an output by a microphone, not shown, such as in amobile telephone or other communication device. In the example shown,such an analog voice/audio signal S_(MC) is in particular a differentialanalog signal to be provided to inputs IN_(P) and IN_(N), of theprocessing system 20.

The system 20 includes an integrated programmable gain amplifier circuit22, for receiving at input the analog signal S_(MC). The circuit 22includes an operational amplifier and a gain setup network 23 havingresistive and selection elements, which interact with the resistivecontrollable elements in order to set, i.e., program, the gain of theintegrated amplifier circuit 22. In the particular example shown, sincethe analog signal S_(MC) is directly provided at output by themicrophone, the integrated programmable gain amplifier circuit 22 istypically called a pre-amplifier circuit.

The gain setup network 23 also includes capacitor elements for defining,together with the resistive elements of the gain setup network and withthe operational amplifier an anti-aliasing filter of the active RC type.The programmable gain amplifier circuit, or pre-amplifier, 22 istherefore able to output an amplified and filtered signal S_(AF).According to an embodiment, above the active RC anti-aliasing filter isa two pole low pass filter.

The system 20 also includes an analog-digital converter 4, which ispreferably provided on the same integrated circuit of the integratedamplifier circuit 22, for receiving at the input the amplified andfiltered analog signal S_(AF), and providing at the output the digitalsamples D_(out), of the signal. Preferably, the analog-digital converter4 is a switched capacitance sigma-delta converter, for example a 16 bitconverter.

In FIG. 3, a first embodiment of the integrated programmable gainamplifier circuit 22 is shown, in a particular example, wherein thecircuit 22, unlike the programmable and differential amplifier circuit22 of FIG. 2, is of the single-ended type. The integrated programmablegain amplifier circuit 22 includes an input terminal IN for receiving atinput the analog signal S_(MC) and an output terminal OU_(P) at whichthe amplifier circuit 22 provides a filtered and amplified analog signalS_(AF).

The integrated amplifier circuit 22 includes an operational amplifier 33preferably of the low noise type (operational amplifier LNA).

The operational amplifier 33 includes an inverting input 34, a noninverting input 35 connected to ground, and an output 36.

The integrated amplifier circuit 22 includes a gain setup networkNRC_(IN), NRC_(F), including resistive elements, or resistors, R₁-R₄ andRf₁-Rf_(n) and selection elements S₁-S_(n), which interact with theresistive elements and are controllable for example by a logic signaloutput by a suitable register (not shown), for setting up the gain ofintegrated amplifier circuit 22. The selection elements S₁-S_(n) arepreferably CMOS switches. According to a modification, the selectionelements S₁-S_(n) are MOS switches, either re-channel (N-ch) orp-channel (P-ch).

The gain setup network NRC_(IN), NRC_(F), includes a first networkNRC_(IN), or input network NRC_(IN), which is connected on the inputside to the operational amplifier 33. In this example it is connected tothe inverting input 34 of operational amplifier 33. A second networkNRC_(F), or feedback network, is connected between the output 36 andinput 34 of operational amplifier 33. In the example of FIG. 3, thefeedback network NRC_(F) is connected between the output 36 and input 34of operational amplifier 33 and therefore defines the feedback loop ofsuch operational amplifier 33.

The selection elements S₁-S_(n) allow varying the ratio between thetotal resistance of feedback network NRC_(F) and total resistance ofinput network NRC_(IN), therefore allowing programming the gain ofamplifier circuit 22. In an advantageous embodiment, the selectionelements S₁-S_(n) are included in the feedback network NRC_(F),therefore allowing the setup of value of total resistance over thefeedback loop of operational amplifier 33.

Advantageously, the gain setup network NRC_(IN), NRC_(F) also includescapacitor elements for defining, along with the resistive elements R₁-R₄and Rf₁-Rf_(n) and the operational amplifier 33, an active RCanti-aliasing filter. Preferably, such filter is a second order activeRC filter. Therefore it can be deduced that the integrated programmablegain amplifier circuit 22 is an amplification and filtering circuit.

Preferably, the input network NRC_(IN) includes one or more capacitorelements C₁-C₃ and one or more resistive elements R₁-R₄ for definingtogether an RC filter, more preferably a single-pole filter, provided atthe input of operational amplifier 33. In a particularly advantageousembodiment, such RC filter is of the distributed kind, so that the inputnetwork NRC_(IN) has many RC cells, respectively (R₁, C₁), (R₂, C₂), and(R₃, C₃), which are cascode-connected and preferably include resistiveelements R₁, R₂, R₃ with the same resistance value and capacitorelements C₁, C₂, C₃ having the same capacitance. This allows theoperational amplifier 33 to have at input a single-pole filter which ismore selective with respect to the RC filter, which is only providedwith a single RC cell.

In the particularly preferred embodiment of FIG. 3, the input networkNRC_(IN) includes three cascode-connected RC cells, and a resistance R₄connected between the last cell R₃, C₃ and input 34, which is aninverting input in this example, of the operational amplifier 33.

The feedback network NRC_(F) includes a plurality of RC cells, which areconnected in a cascode (Rf₁, Cf₁), (Rf₂, Cf₂), . . . , (Rf_(n), Cf_(n))and each of them is associated with a respective selection element S₁, .. . S_(n), so that selectively closing only one at a time of theselection elements S₁, . . . S_(n) it is possible to vary the number ofRC cells that are cascode-connected between the output 36 and input 34of operational amplifier 33, and therefore vary the number of RC cellsthat define the feedback loop of the operational amplifier 33. Forexample, by activating, i.e., closing the switch, of selection elementS₁ and leaving all other selection elements S₂, . . . S_(n) open, thefeedback loop will only include one RC cell, i.e., the RC cell (Rf₁,Cf₁) connected between the output 36 and the input 34 of the operationalamplifier 33. In an analogous way, by activating, i.e., closing theswitch, of selection element S₂ and leaving all other selection elementsS₁ and S₃, . . . S_(n) open, the feedback loop will have twocascode-connected RC cells, i.e., RC cell (Rf₁, Cf₁) and cell (Rf₂, Cf₂)connected between the output 36 and the input 34 of the operationalamplifier 33. In this way, since, based on the particular activatedselection element, it is possible to determine the total resistanceretroactively connected to the operational the amplifier 33, it ispossible to set the gain of amplifier circuit 22.

In a particularly advantageous embodiment, the resistance andcapacitance values of capacitive and resistive elements of the feedbacknetwork NRC_(F) are defined in the design phase so that, independentlyfrom the number of RC cells (Rf₁, Cf₁), (Rf₂, Cf₂), . . . , (Rf_(n),Cf_(n)) which are cascode-connected between the output 36 and input 34of operational amplifier 33, the feedback network NRC_(F) defines theother one of the two poles of the anti-aliasing filter, the feedbacknetwork NRC_(F) being describable by a passive RC filter with a singlepole having a substantially constant cut-off frequency. In order toachieve this, it is sufficient that in the RC cells the capacitancevalues are proportionally scaled with respect to the value of associatedresistances.

In this way, advantageously, the cut-off frequency of the anti-aliasingfiltering implemented by the integrated amplifier circuit 22 may be setat a substantially constant value with respect to a set gain variation.Based on these specifications, when the cut-off frequency of RC filterdefined by the feedback network NRC_(F) is set, and once the desiredpitch and range of gain variation are defined, one skilled in the artmay easily determine, in the design phase, the number of RC cells of thefeedback network NRC_(F) and the values to be selected for thecapacitive and resistive elements of such network NRC_(F), which may becompletely different from one another.

With reference to FIG. 4, according to a modified embodiment, theintegrated programmable gain amplifier circuit 22 includes a feedbacknetwork NRC_(F) having a plurality of cascode-connected RC cells,wherein at least one of the RC cells has a capacitive element, at leasttwo resistive elements, which may be independently series-connectedbetween the input 34 and output 36 of operational amplifier 33 by meansof respective independently controllable selection elements, which areprovided in the feedback network NRC_(F). In the particularly preferredembodiment of FIG. 4, with the exception of the first RC cell Rf₁, Cf₁,all the remaining RC cells of the feedback network NRC_(F) include acapacitive element and two resistive elements, which may be connectedbetween the output 36 and input 34 of operational amplifier 33 by meansof respective independently controllable selection elements. In thisregard, it is to be noted that in FIG. 4 the second RC cell (Rf₂₁, Rf₂₂,Cf₂) includes a capacity Cf₂ and two series connected resistors Rf₂₁,Rf₂₂, which may be independently connected between the input and outputof operational amplifier by means of respective selection elements S₂₁,S₂₂ in order to vary the gain of integrated programmable gain amplifiercircuit 22. It is to be noted that also in the integrated amplifiercircuit 22 of FIG. 4, in order to set the gain value, it is necessary toclose only one of the selection elements S₁, . . . S_(n2).

It is to be noted that by providing a plurality of resistances for eachRC cell, as described above, and accepting a reasonably lower precisionin maintaining a constant cut-off frequency in the feedback networkNRC_(F), and in general of anti-aliasing filtering implemented in theintegrated programmable gain amplifier circuit 22, it is possible toprovide an integrated programmable gain amplifier circuit 22 that,having less capacitive elements than the circuit of FIG. 3, requires areduced surface area with respect to the latter.

FIG. 5 shows an integrated programmable gain amplifier circuit 22 thatis very similar to the one shown in FIG. 3, wherein the circuit is ofthe fully-differential type.

In this case, the operational amplifier 33 is of a differential type,and has two inputs 34, 35 and two output 36 _(P), 36 _(N). Thegain-setting network includes an input network NRC_(IN) and two feedbacknetworks NRC_(F) _(—) _(P) and NRC_(F) _(—) _(N).

The input network NRC_(IN) has three cascode-connected RC cells, whichare similar to those previously described, wherein, since the two inputs34, 35 of the operational amplifier 33 are virtual ground nodes, each ofsuch RC cells has one capacitive element and two resistive elements. Theinput network NRC_(IN) defines a single pole RC filter. Each of the twofeedback networks NRC_(F) _(—) _(P) and NRC_(F) _(—) _(N) embodies amulti-cell single pole RC filter and the capacitive and resistiveelements are defined so that the cut frequency of the anti-aliasingfiltering is substantially constant with respect to a gain variation inthe integrated amplifier circuit 22.

All in all, the integrated amplifier circuit 22 is a programmable gainintegrated differential amplifier circuit, which is also ananti-aliasing differential active filter of the second order, of the lowpass type. It is to be noted that in order to set the gain in theintegrated circuit 22 of FIG. 5, only one of the selection elements S₁,. . . S_(n) of feedback network NRC_(F) _(—) _(P) has to be activated,for example by a control circuit (not shown) through a register, forexample activating the selection element S₁, and the correspondingselection element, i.e., S₁, and only this one in the other feedbacknetwork NRC_(F) _(—) _(N).

Finally, FIG. 6 shows a modification of amplifier circuit of FIG. 5,wherein, in analogy to the description of the circuit of FIG. 4, for thesingle-ended case, the number of capacitive elements to be provided inthe feedback networks NRC_(F) _(—) _(P) and NRC_(F) _(—) _(N) may bereduced.

From the above description, it is therefore possible to understand how aintegrated programmable gain amplifier circuit 22 of above the typeachieves the intended objects, allowing a reduction of surface area, byintegrating the programmable gain amplification function and theanti-aliasing filtering function, and at the same time maintaining asubstantially constant cut frequency of the anti-aliasing filtering,with respect to a variation in the particular gain to be set.

Obviously, the skilled in the art, in order to satisfy contingent andspecific needs may introduce various modifications and variations to theabove integrated programmable gain amplifier circuit and analog signalprocessing system, which all remain within the protection scope of thedisclosure, as defined in the following claims.

The various embodiments described above can be combined to providefurther embodiments. All of the U.S. patents, U.S. patent applicationpublications, U.S. patent applications, foreign patents, foreign patentapplications and non-patent publications referred to in thisspecification and/or listed in the Application Data Sheet areincorporated herein by reference, in their entirety. Aspects of theembodiments can be modified, if necessary to employ concepts of thevarious patents, applications and publications to provide yet furtherembodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

1. An integrated programmable gain amplifier circuit, for receiving atan input an analog signal, the circuit comprising an operationalamplifier and a gain setup network comprising resistive elements andselection elements, that are controlled to setup gain of the amplifiercircuit, the gain setup network comprising capacitive elements, fordefining, together with the resistive elements and the operationalamplifier, an anti-aliasing filter of the active RC type.
 2. Theintegrated programmable gain amplifier circuit of claim 1, wherein theanti-aliasing filter is a second order RC filter.
 3. The integratedprogrammable gain amplifier circuit of claim 1, wherein the gain setupnetwork comprises a first network, or input network, which is connectedto an input (34) of the operational amplifier and a second network, orfeedback network, which is connected to an output of the operationalamplifier and the input of the operational amplifier, and wherein theselection elements allow varying the ratio between total resistance ofthe feedback network and total resistance of input network, therebyallowing programming of amplifier circuit gain.
 4. The integratedprogrammable gain amplifier circuit of claim 3, wherein the selectionelements are included in the feedback network, to selectively determinea total series resistance value between the output and the input.
 5. Theintegrated programmable gain amplifier circuit of claim 4, wherein thefeedback network comprises a plurality of RC cells connected to eachother to form a cascode, each one of the RC cells associated with arespective selection element so that by selectively activating only oneof the selection elements, one at a time, the number of RC cellscascode-connected between the output and the input of the operationalamplifier can be varied.
 6. The integrated programmable gain amplifiercircuit of claim 5, wherein the plurality of cascode-connected RC cellscomprises at least an RC cell comprising a capacitive element, at leasttwo resistive elements, and at least two independently controllableselection elements that are each associated with one of the at least tworesistive elements, the at least two resistive elements independentlyseries-connectable between the input and output of the operationalamplifier by means of the independently controllable selection elements.7. The integrated programmable gain amplifier circuit of claim 3,wherein the input network comprises one or more capacitive elements andone or more resistive elements, for defining, as a whole, an RC filterthat is positioned at the input of operational amplifier.
 8. Theintegrated programmable gain amplifier circuit of claim 7, wherein theRC filter is of the distributed type, so that the input networkcomprises a plurality of cells cascode-connected to each other andprovided with resistive elements having the same resistance andcapacitive elements having the same capacitance.
 9. The integratedprogrammable gain amplifier circuit of claim 1 wherein the resistiveelements and the capacitive elements have resistance and capacitancevalues so that a cut-off frequency of the anti-aliasing filter isconstant with respect to a gain variation of the amplifier circuit. 10.The integrated programmable gain amplifier circuit of claim 1, furthercomprising an analog-digital converter (4), which is connected at theoutput of the operational amplifier.
 11. A circuit for processing ananalog signal, the circuit comprising: an anti-aliasing filter of theactive RC type that comprises: an amplifier; and a gain network thatcomprises an input network coupled to a first input of the amplifier,and a feedback network coupled between an output of the amplifier and asecond input of the amplifier, the feedback network comprising aplurality of RC cells coupled in cascode, each RC cell coupled to arespective selection switch to selectively couple each of the pluralityof RC cells in the cascode connection.
 12. The circuit of claim 11,comprising an analog-to-digital converter coupled to an output of theamplifier.
 13. The circuit of claim 11, wherein the programmable gainamplifier is a fully differential amplifier comprising a second feedbacknetwork coupled between a second output of the amplifier and the firstinput of the amplifier.
 14. A mobile communications device, comprising:an anti-aliasing filter of the active RC type that comprises: anamplifier; and a gain network that comprises an input network coupled toa first input of the amplifier, and a feedback network coupled betweenan output of the amplifier and a second input of the amplifier, thefeedback network comprising a plurality of RC cells coupled in cascode,each RC cell coupled to a respective selection switch to selectivelycouple each of the plurality of RC cells in the cascode connection. 15.The mobile communications device of claim 14, comprising ananalog-to-digital converter coupled to an output of the amplifier. 16.The mobile communications device of claim 14, wherein the programmablegain amplifier is a fully differential amplifier comprising a secondfeedback network coupled between a second output of the amplifier andthe first input of the amplifier.